The invention relates generally to optical switching elements and more particularly to components in switches in which optical coupling among waveguides is determined by manipulating fluid.
In the past, telecommunications and data communications networks have traditionally relied on electrical signals transmitted electrically on conductive lines. As higher and higher data exchange rates are required, conductive lines are no longer sufficient and increasingly the data is transmitted through the use of optical signals through optical fibers. Equipment for efficiently generating and transmitting the optical signals has been designed and implemented, but the manufacturability of optical switches for use in telecommunications and data communications networks is problematic.
Fouquet et al. (U.S. Pat. No. 5,699,462), which is assigned to the assignee of the present invention, describes a switching matrix that is used for routing optical signals from any one of a number of parallel input optical fibers to any one of a number of parallel output optical fibers.
Referring now to FIG. 1 (PRIOR ART), therein is shown an isolated optical switching element 10 formed on a substrate 12. The substrate 12 is of silicon or silica. The optical switching element 10 includes planar waveguides defined by a lower cladding layer 14, a core 16 and an upper cladding layer 18. The core 16 is primarily silicon dioxide, but other materials that affect the index of refraction of the core may be used. The cladding layers 14 and 18 are formed of a material having a refractive index that is substantially different from the refractive index of the core material, so that optical signals are guided along the core material.
In the manufacturing process, the core 16 is patterned to define an input waveguide 20 and an output waveguide 26 of a first waveguide path and to define an input waveguide 24 and an output waveguide 22 of a second waveguide path. The upper cladding layer 18 is then deposited over the core 16. A trench 28 is etched through the core 16 at the intersection of the first and second waveguide paths and the two cladding layers 14 and 18 to the substrate 12. The waveguide paths intersect the trench 28 at an angle of incidence greater than the critical angle of total internal reflection (TIR) when the trench 28, which is filled with a fluid having a refractive index that closely matches the refractive index of the waveguides, contains a bubble.
Thus, TIR diverts light from the input waveguide 20 to the output waveguide 22, unless an index-matching fluid is located within the gap between the aligned waveguides 20 and 26. The trench 28 is positioned with respect to the four waveguides 20, 26, 24, and 22 such that one sidewall of the trench 28 passes through or is slightly offset from the intersection of the axes of the waveguide paths.
Referring now to FIG. 2 (PRIOR ART), therein is shown a plurality of the optical switching elements 10 in a 4 times 4 matrix 32. In the 4 times 4 matrix 32, any one of four input waveguides 34, 36, 38 and 40 may be optically coupled to any one of four output waveguides 42, 44, 46, and 48. The switching arrangement is referred to as xe2x80x9cnon-blocking,xe2x80x9d since any free input waveguide can be connected to any free output waveguide regardless of which connections have already been made through the switching arrangement. Each of the sixteen optical switches has a trench that causes TIR in the absence of an index-matching liquid at the gap between collinear waveguides, but collinear waveguides of a particular waveguide path are optically coupled when the gaps between the collinear waveguides are filled with the refractive index-matching fluid. Trenches in which the waveguide gaps are filled with fluid are represented by fine lines that extend at an angle through the intersections of optical waveguides in the array. On the other hand, trenches in which the index-matching fluid is absent at the gaps are represented by broad lines through a point of intersection.
For example, the input waveguide 20 of FIGS. 1 and 2 (PRIOR ART) is in optical communication with the output waveguide 22, as a result of reflection at the empty gap of the trench 28. Since all other cross points for allowing the input waveguide 34 to communicate with the output waveguide 44 are in a transmissive state, a signal that is generated at the input waveguide 34 will be received at output waveguide 44. In like manner, the input waveguide 36 is optically coupled to the first output waveguide 42, the third input waveguide 38 is optically coupled to the fourth output waveguide 48, and the fourth input waveguide 40 is coupled to the third output waveguide 46.
There are a number of available techniques for changing an optical switch of the type shown in FIG. 1 from a transmissive state to a reflective state and back to the transmissive state. One method of changing states involves forming and eliminating the gap by forming and removing bubbles in a refractive index-matching liquid. A plurality of heating elements are used where the application of heat to a trench forms the bubble and the removal of the heat causes the bubble to collapse. The heating elements are activated by external signals on leads on a reservoir substrate.
Essentially, the refractive index-matching liquid resides within the trench in the waveguide paths until a bubble is formed to create an index mismatch and cause light to be reflected at the sidewall of a trench. Collapsing the bubble returns the switch to the transmissive state. The refractive index-matching liquid is supplied through reservoirs above and below the waveguide paths and is resupplied from outside of the 4 times 4 matrix 32 through the substrate 12.
The passages for filling the matrix with the refractive index-matching liquid are located in the reservoir substrate and require machined right angle passages which are extremely difficult to manufacture resulting in high manufacturing costs.
Further, thermal expansion mismatch has been a problem which leads to bowing of the waveguides and also the loss of the hermetic sealing to the bottom reservoir.
Even further, it has been determined that the relatively low thermal conductivity of the current substrate materials slows the operation of the optical switching elements because heat is retained and the bubble does not collapse quickly.
These problems are becoming bottlenecks in the development of the optical switch and the solutions to these problems have eluded those skilled in the art.
The present invention provides an optical cross-connect switch which includes a planar lightwave circuit having a number of optical waveguides and trenches. The planar lightwave circuit has a first waveguide and a second waveguide that intersect the trench such that optical coupling between the first and second waveguides is dependent upon a presence of a fluid at an intersection of the trench with the first and second waveguides. A mounting base, having a channel and through openings provided therein, has fill tubes disposed in the groove and through openings of the mounting base. A reservoir pedestal having through openings provided therein and bonded to the reservoir pedestal with the fill tubes extending through the through openings therein to form a reservoir substrate. A bubble-forming element is disposed on a first surface of the reservoir pedestal, including patterning a plurality of leads on the first surface and a sealing/bonding ring bonds and seals the planar lightwave circuit and the reservoir substrate to form a reservoir having the bubble-forming element and planar lightwave circuit aligned such that the reservoir is in fluid communication with the fill tubes through the reservoir substrate and the trench in the planar lightwave circuit which is in thermal communication with the bubble-forming element on the first surface of the reservoir pedestal. The optical cross-connect switch is easily and inexpensively formed, and further is not subject to thermal conductivity mismatch or low thermal conductivity problems.
The present invention further provides a method of fabricating an optical cross-connect switch which includes providing a planar lightwave circuit having a plurality of optical waveguides and a trench provided therein. The planar lightwave circuit includes a first waveguide and a second waveguide that intersect the trench such that optical coupling between the first and second waveguides is dependent upon a presence of a fluid at an intersection of the trench with the first and second waveguides. A mounting base having a channel and through openings provided therein is provided with fill tubes disposed in the groove and through openings of the mounting base. A reservoir pedestal having through openings provided therein is provided and bonded to the mounting base with the fill tubes extending through the through openings therein to form a reservoir substrate. A bubble-forming element is formed on a first surface of the reservoir pedestal, including patterning of a plurality of leads on the first surface. A sealing/bonding ring is provided and used to bond and seal the planar lightwave circuit and the reservoir substrate to form a reservoir. The bubble-forming element and planar lightwave circuit are aligned such that the reservoir is in fluid communication with the fill tubes through the reservoir substrate and the trench in the planar lightwave circuit is in thermal communication with the bubble-forming element on the first surface of the reservoir pedestal. The optical cross-connect switch is easily and inexpensively formed, and further is not subject to thermal conductivity mismatch or low thermal conductivity problems.
The above and additional advantages of the present invention will become apparent to those skilled in the art from a reading of the following detailed description when taken in conjunction with the accompanying drawings.